Even on days when the temperature outside doesn’t seem that hot, the temperature inside a car can climb with deadly speed, experts warn.

So even on days when the temperature won’t hit record highs, it’s important to remember to never leave a child alone in a car.

Jan Null, a San Francisco meteorologist who has researched heat stroke deaths among children left in cars, studied nearly 500 child heat stroke deaths recorded between 1998 and 2011 in the U.S.

In just over half the fatalities, parents or caregivers reported forgetting that the child was in the vehicle, Null found.

So far this year, at least 11 children in the U.S. have died of heat stroke after being left in vehicles, Null said.

The most recent fatality came July 9 in Arizona. The temperature outside was 113 degrees. On July 7, a 4-month-old died in Greenfield, Ind.

“In that fatality in Indiana, the temperature outside was 105 degrees. You add 40 or 45 degrees to that, you’re pushing 150 degrees inside that car,” he said.

In many cases, a change in routine was linked to the fatal event. Sometimes, it was something as seemingly small as a switch in which parent was supposed to drop the child off at day care, said Jessica Saunders, Safe Kids Greater Dayton coordinator and community relations manager at Children’s Medical Center of Dayton.

“I have an 18-month-old,” Saunders said. “I know how crazy my mind is in the morning. I tell parents to admit that it could happen to you so they can take the steps to prevent it from ever happening to you.”

Safe Kids USA this summer kicked off a national campaign reminding parents of the dangers of leaving their children in hot cars. The key message of the campaign is, “Look before you lock.”

Several strategies can help parents remember their young children are in the car:

• Baby sitters or day care providers should call parents if a child isn’t dropped off at the usual time. And parents should call each other to check that the child was dropped off, especially if there’s been a switch in the morning routine, Saunders said.

• Put your briefcase, purse or, better yet, cellphone, next to the car seat. If you forget to drop the child off, you’ll know as soon as you reach back to get your stuff.

• Put a standing reminder on your daily calendar so it pops up to remind you that you were supposed to drop off the child.

• Some smartphones offer apps that remind parents their children are in the car.

Cars act like mini-greenhouses and heat up rapidly, even on fairly cool days, Null said.

In the first 10 minutes after the car is parked and the door is locked, the temperature inside a car climbs 19 degrees above the outdoor temperature, he said.

“On an 80-degree day, the temperature in that car hits 99 degrees in about 10 minutes. So even if you have the AC blasting, as soon as you close that door, the temperature starts heating up,” Null said.

After an hour, on average, the temperature inside the car rises to about 43 degrees hotter than the temperature outside. The temperature inside the car generally peaks at about 45 to 50 degrees hotter than the temperature outdoors, Null said.

Leaving the windows open a few inches only lowers the temperature by a few degrees, he said. “The few days I looked at, the temperature in the car got up to 130 degrees. With the windows cracked, they were like 127. That’s not enough to make a difference; not enough to make it survivable.”